From early on, infants show a preference for infant-directed speech (IDS) over adult-directed speech (ADS), and exposure to IDS has been correlated with language outcome measures such as vocabulary. The present multi-laboratory study explores this issue by investigating whether there is a link between early preference for IDS and later vocabulary size. Infants’ preference for IDS was tested as part of the ManyBabies 1 project, and follow-up CDI data were collected from a subsample of this dataset at 18 and 24 months. A total of 341 (18 months) and 327 (24 months) infants were tested across 21 laboratories. In neither preregistered analyses with North American and UK English, nor exploratory analyses with a larger sample did we find evidence for a relation between IDS preference and later vocabulary. We discuss implications of this finding in light of recent work suggesting that IDS preference measured in the laboratory has low test-retest reliability.

Spelke's What Babies Know masterfully describes infants’ impressive repertoire of core cognitive concepts, from which the suite of human knowledge is eventually built. The current commentary argues for the existence of a core concept that Spelke claims preverbal infants lack: social goal. Core social goal concepts, operative extremely early in human development, underlie infants’ basic abilities to interpret and evaluate entities within the moral world; such abilities support claims for a core moral domain.

Grossmann posits that heightened fearfulness in humans evolved to facilitate cooperative caregiving. We argue that three of his claims – that children express more fear than other apes, that they are uniquely responsive to fearful expressions, and that expression and perception of fear are linked with prosocial behaviors – are inconsistent with existing literature or require additional supporting evidence.

Yarkoni's analysis clearly articulates a number of concerns limiting the generalizability and explanatory power of psychological findings, many of which are compounded in infancy research. ManyBabies addresses these concerns via a radically collaborative, large-scale and open approach to research that is grounded in theory-building, committed to diversification, and focused on understanding sources of variation.

Whereas Phillips and colleagues argue that knowledge representations are more basic than belief representations, we argue that an accurate analysis of what is fundamental to theory of mind may depend crucially on the context in which mental-state reasoning occurs. Specifically, we call for increased study of the developmental trajectory of mental-state reasoning within socially evaluative contexts.